Horny Ghosts
by tj1919
Summary: Dipper and Mabel have some invisible fun. Lemon
1. Chapter 1

Via AFF user PineWreaths

Dipper groaned as he began to slowly awaken. The dream he'd been having was heavenly, imagining a warm mouth arousing him, and-

Funny; Mabel doesn't normally make this much noise in the dreams.

Dipper looked up, and gave a slight shriek of embarrassment.

His cock was out, erect and gently swaying. He had been sitting in the lazyboy in the living room of the Mystery Shack, and drifted off to sleep while another rerun of "The Duchess Approves" played on the TV.

Now, the TV was off, but the overhead light was on, revealing his pants had, somehow, slid to the floor, and he was fully visible should Grunkle Stan come wandering through the door at any minute. The warm pleasurable feelings from the dream were still incredibly strong, but Dipper was more worried about his Grunkle than he was about finishing off a happy daydream. He frantically looked around and grabbed a pillow, jamming it down over his exposed erection.

That caused a very unexpected sensation as something blocked the pillow; He felt the warm dream-mouth abruptly cough and partially bite his penis, causing him to yelp. The sucking sensation withdrew sharply, and as Dipper finally managed to squish the erection flat under the spare pillow.

Ok, definitely not dreaming. The dream-girls never bit Little Dipper.

"Ow, Dip, what gives?"

Mabel's voice emanated from midair directly in front of Dipper, but he couldn't see anything. He squinted, pinching himself mentally to see if he was still asleep, when there was a quiet sound of feet on carpet, and a warm mouth enveloped his.

He recognized her taste immediately; Mabel still preferred vanilla-bean lipgloss, and the enthusiasm her tongue had for his was certainly familiar as well. The kiss broke, and Dipper flopped back into the chair, now with more questions than answers as he realized his sister was honest-to-goodness invisible.

Mabel must have seen his look, because a giggle emanated from thin air, and he could feel a weight settle on the arm of the chair, as she said in a conversational tone "Brobro, Gravity Falls has a lot more weirdness than just what Grunkle Ford had in his journals."

He cocked his head, intrigued and his heart pounding for a variety of reasons, one of which he was fairly sure was naked and less than a few inches from him. He reached out a tentative hand, gently brushing it where he expected her to be, and-

There.

His hand touched flesh, and he could hear her shivering breath as he ran a hand up the side of her bare thigh. "Well now, mister, I thought after the-ahah," her breath caught as Dipper's hand shifted to the inside of her thigh and continued its ascent. "I thought after the head-pillow earlier that you weren't interested."

Dipper chuckled, saying in an equally low voice "Well, I hadn't particularly imagined that I'd wake up to getting a blowjob from my sister's ghost," his voice taking on a tinge of teasing as his hand almost found its mark.

Now that they were both in college, visiting Gravity Falls was a solidified tradition each summer, and Dipper and his sister had been enjoying each other's "company" most nights for years. As a result, even essentially blind, he was fully familiar with exactly how best to tease and please Mabel, as a combination revenge and thank-you for the invisible sucking.

Mabel's voice continued to be calm, but with the occasional hitch in her voice as his hand lingered and caressed at the top of her thigh. "There's-ah-there's a cave, on the far s-side of the lake," she said, stuttering a bit as one finger lingered daringly close to her before returning. "It's got a waterfall thing, one that runs over a set of these really pretty silver crystals. I was-ooh, mhmmm-I was following some squirrels, and could hear them chittering but couldn't see them. Ducked under the water to get a better look, and bam! Invisible Mabel."

He chuckled again, saying "It's still three hours until dinner; Let's go take a look-see, shall we?" He removed his hand, causing his twin to give a whimper of disappointment, but as he stood and reached one hand for his pants, he could hear her move, and felt a shove to the center of his chest. He fell backwards into the chair with a "Whoof!" of air.

Suddenly, the smell of Mabel's skin was very close, and he could hear her whisper huskily into his ear "Oh, we have hours before we run out of daylight. Just sit tight for a second, little bro, and let Little Dipper do his thing."

With that, he felt her hand grab his shaft, and with a few strokes he was rock-hard again, the confusion-induced flaccidness melting away as his breathing sped up, matching the little breaths he could hear his sister making right next to him.

Then, with a happy moan, he could feel Mabel maneuver his cock, rubbing it teasingly along her once, twice, three times, eliciting a pleading whimper from him before she sat heavily, engulfing him entirely as she let out a groan of pleasure.

Dipper reached forward, feeling the smooth skin of her back, and wrapping around to cup her breasts, teasing the nipples gently as he thrust his hips upwards. Mabel let out an appreciative "MHMmph oh yessiree," her hips slamming back down to meet his thrusts, each causing her to let out a little involuntary blissful "Ah!"

He opened an eye, and his jaw dropped. For all intents and purposes, it looked like Dipper had his hands forward like a hug-oriented zombie, pants around his ankles as he thrust into midair. He let out a snort of amusement at the sight between groans of effort, and Mabel slowed to rock against him. He could feel her shift, probably to turn and look towards him.

"What-what is it, Dip?" she said, panting slightly as he thrust again. He let go of her with one hand, gesturing around at how he was splayed out in the chair, and she let out a snicker as she saw how odd it was too. "Well, brobro, it's probably a good thing Grunkle-"

"Stan? Stanley, where in the blazes are you?"

Mabel let out a squeak as Ford's voice cut her off from the direction of the vending machine. Dipper let out another whimper, this time of frustration as her amazing wetness slid off of him, leaving Little Dipper out in the cold to shiver as a breeze blew through the house.

He could feel Mabel brush past him, standing between the chair and the wall with her hip against his hand as Dipper re-shoved the pillow on top of his crotch. He had just enough time to whisk a blanket over the pillow and his obvious lack of pants, slouching back as Grunkle Ford came stomping into the room, looking around peeved.

Dipper leaned forward, saying "What's wrong Grunkle Ford?" as he stretched, acting as though he had just awoken from a nap. Which was almost true, technically.

However, as he stretched out, his hand contacted Mabel's hip, and he could feel her shiver, although she didn't move and said nothing with their Grunkle in the same room. Dipper resisted the urge to smile, ideas for mischief already running rampant through his mind.

Ford had pulled out his new journal, the ink for the '4' on the front still clear and unfaded, and leaned against the wall as he checked something, snapping it shut and looking around in annoyance again. "Damn it, Stanley was supposed to meet me half an hour ago for a fishing trip, but surprise, surprise,he's late as usual."

Dipper kept his attention focused on his Grunkle, nodding attentively while stretching again. This time, his fingers stretched too, which, by sheer coincidence and certainly not careful memorization of where he'd felt his sister standing, caused one stretching finger to run along the edge of her lips and past her clit.

Mabel let out a muffled quiet "Eeph!" the sound suddenly muffled, likely by her hand. Ford looked up, eyebrow raised in confusion, but Dipper just coughed awkwardly, finishing his stretch and again brushing his fingers over a his sister's clit as he said "Sorry; I guess I make weird noises when I stretch."

Ford considered this for a second, then chuckled. "Ah, well, you do sneeze like a kitten," he said jovially, the reminder making Dipper flush with embarrassment.

It was good that his embarrassment already gave him a flush, because he could feel his ears ignite as Mabel grabbed his dangling hand, angling it slightly so one finger was nestled just outside her. He could feel the aching tension in her body, and he gently reached a finger up into her, making a come-hither gesture as he could feel her shudder, the weight of a hand suddenly on the chair arm next to him as she supported herself.

Ford was engrossed in his journal, looking at a sketch in it and grumbling before erasing most of it and reworking it. Dipper grinned slightly, when he could feel his sister shudder again, and clench around his finger, and to Dipper's sudden terror, let out an audible "Dip!"

Ford looked up sharply as Dipper could feel his sister stumble off of him and towards the stairs. His Grunkle opened his mouth to say something when Mabel called down, sounding like she was about halfway down the stairs. "Dipper?"

Dipper squinted in confusion, before relaxing back against the wall with a snort. Mabel called down again, saying "Dipper, I wanted to show you that cave and those squirrels I was telling you about earlier. Will you be ready to go after I shower?"

Clearing his throat, Dipper called back "Uh, yeah, no problem," thanking everything that was holy that Ford had wandered into the hallway. His Grunkle snorted out "Finally," before stepping through the door to the awaiting Stanley Mobile as Grunkle Stan honked the horn. Before stepping out, he turned to Dipper, saying "Now remember, dinner is at seven, and if you're late, you get leftovers, if Soos leaves you any."

Dipper nodded, silently wishing his Grunkle out the door sooner, and the moment after Ford nodded and slammed shut the door, Dipper stood and yanked on his pants. He had just zipped up the fly when Ford slammed the door back open, causing Dip to let out a little shriek of surprise.

"Forgot the tackle box," he said, giving Dipper a smile and a thumbs up as he grabbed a little squat green box, and then the door slammed and Dipper could hear the car peel off.

A few minutes later, a damp and now thoroughly-visible Mabel came down, shrugging on a sweater and squeezing a few remaining drops of water out of her hair. She gave him a grin, her face red from more than just the hot water if Dipper had to guess, but she did have a note of relief in her voice.

"Man, god thing the invisiwater washes out; I was worried for a minute that it woulda been permanent, and that would have seriously crimped my fashion-display abilities," she said, posing model-style for her brother as he let out a laugh.

Giving him a wink, she jerked a thumb towards the door, before waving her hands and saying "So Dipper, you ready to become a spooooky ghooooost?," her voice wavering mysteriously for added ghostly impact.

Dipper smiled, and said in a low, teasing voice "More like horny ghosts, Mabes," as he strode past her her, cupping her ass as he passed. She let out a little squeal, slapping his hand away before dashing out the door.

"Last one there's a rotten egg," she called back, giggling gleefully at Dipper's protesting reply.

"No fair, Mabel, I don't even know where this thing is," he yelled, chasing his twin into the trees.


	2. Chapter 2

It only took the twins a half-hour of hiking and running through the trees to get to the edge of the lake Mabel had described. Sure enough, the cave was difficult to see, secluded and partially hidden by the hanging rootmass of an overturned spruce.

She ducked inside and her brother followed, Dip ducking his head to avoid the stony ceiling. Running water echoed from inside, but the interior was surprisingly well-lit; Water running through the soft rock of the cave had eroded little windows, most no bigger than a fist, in the mottled wall, and beyond Dipper could distantly make out the shape of the lake and a few tiny fishing boats.

His attention was caught by his sister giggling and disrobing; As usual, she hadn't tossed on a bra, instead opting to take advantage of the volume of the sweater as concealment of the fact. Dipper grinned, appreciating the view as she laughed and splashed over to the small waterfall, the water almost making a musical tinkling as it hit the shallow pools below.

Dipper couldn't help but stare at the beautiful crystals, and desperately wish he'd brought his own still-unfinished journal to do some sketches. It looked like they'd been formed of glass, their reflection so clear he could see his own birthmark clearly on his face, even with the water running over the surface of the gem.

He couldn't help but run a finger along the surface of the gem, feeling an odd warmth as he drew his hand back; The fingertip was coated silver, which wispily faded into nothingness, as did the finger.

Dipper was so preoccupied that he hadn't bothered to pay attention as his sister, her arms now invisible up to her elbows, carried over a cupped invisible-handful of water to splash on him.

Dipper jumped backwards, instinctively going to wipe his nose off and-

He missed.

Instead, his invisible hand, merrily screwing with his perception of where it had been, nearly poked him in the eye, causing Dip to swear and flinch as Mabel again splashed him.

Dipper groaned as he could feel his clothes getting soaked with his sister's antics, but when he looked they were wet, but still clearly visible, unlike most of his body.

"Huh? Mabel, our clothes…"

"The effect doesn't work on anything except animals, I think," she said, ducking her head under the stream and causing it to vanish. Her invisible head spluttered and spat excess water, and he could hear her wringing out her hair as she said "Just fold and stick your clothes over by mine, behind that rock that looks like Grunkle Stan's head."

Dipper nodded, pulling off his shorts and shirt, and sticking them, his hat, and finally his boxers behind rock that looked uncannily like their Grunkle's face. As he turned, a pair of pants and sandals went sailing past him, and Mabel, or at least most of her above the knees and below her bust, was standing there with patchily-visible arms on her hips.

Dipper smiled, and said with a murmur "Huh. Weird, but I also can't say I _dislike_ the view." He could hear Mabel's chuckle as she walked towards him a slinky swing in her hips and an odd tense note in her voice as she said "Well, does Little Dipper approve?"

Dip grinned, leaning his hips and his suddenly-straining erection forward as he said "You know he does." He frowned, letting out a breath as Mabel said again with the oddly-tense voice "I _know,_ do I?"

He looked down, letting out a little whimpering groan of frustration as he saw that one of his sister's splashes had granted his penis the gift and curse of invisibility. He could feel the erection, but it currently looked like the region his shorts normally occupied had been crudely cropped out of existence instead.

Mabel's oddly-tense tone finally dissolved into a _"Snrk!"_ and then a guffaw of laughter at the look on her brother's face as he whimpered "But-but-" with a slightly pathetic note in his voice. Then a mottled arm grabbed his, and he was yanked under the waterfall, the rest of him and his sister going transparent as she pulled him in for a kiss.

Dipper closed his eyes as she ran her tongue along his lips with a giggle; It wasn't like looking around was going to do a whole lot anyways, but he could feel her warmth, her shivers as the cold water struck bare skin, her writhe against him.

"Let's finish the job from earlier, brobro," she said huskily as she guided him between her legs, and engulfed him for the second time that night.

This time, Dipper ran a hand down, running a finger along and around her clit as he plunged into her. Water splashed off her shoulders, crudely outlining her form as she moaned his name. Her legs wrapped around his waist, and Dipper removed his hands to cup his sister's ass, leaning her gently back up against the smooth water-worn wall before thrust again.

He ducked his head down to kiss her breasts, but ducking his head around he kept seeming to miss them slightly before he could get a fix on them. Then Mabel shifting her weight keyed him in to her messing with him; That, and her giggling laugh, before Dipper finally caught her and engulfed a nipple, giving her a gentle bite of reprimand as the laugh morphed into a shuddering breath.

Finally, Dipper could begin to feel himself clench; He cursed inwardly a bit, as he was hoping the fun would be able to last a little bit longer, but the blueballs from earlier had other plans. Dipper reached down again, running a finger along her lips before massaging her clit with his thumb, and he was rewarded a few seconds later by the feeling of her clenching, her arms suddenly reaching around him to dig shallow tracks in his back as she sucked in breath through her teeth, making a strained little _"yesyeyyesohgodyesyes-"_

That sound drove Dipper to release, and as she stopped shuddering he came, feeling himself spurting into her and sending Mabel into a second round of shuddering breaths as she clenched and had a second, smaller orgasm. They hugged each other close, Dipper going soft inside her as they felt their breathing slow.

Mabel disengaged first, and Dipper felt her move, a soft wet noise, and-

Dipper had already thrashed away as he felt her sticky thumb across his forehead, swearing and going to wash off his anointed forehead as Mabel began cackling. After cleaning off his head, Dipper splashed back in the direction his sister's giggling was coming from, saying "Hey Mabel, you know how you hate wet willies?"

Mabel's cackle suddenly cut into a voice fraught with concern. "Uh, yeah?"

Dipper had just barely brushed the side of her head when she knocked his hand back with a squeal, laughing as she ran out of the cave. Dipper followed, laughing as well in the clear night air.

The ground turned out to be fairly soft loam, and pretty easy to walk on. He had worried about footwear and protection on their hike in, especially after he found that they wouldn't go invisible like the rest of him, but now it turned out to be a non-issue.

Dipper felt weird being nude, despite being invisible. Mabel had always been the one more at-home with tossing off most or all of her clothes at the end of the day and dancing around home naked as the day she was born when they had the Shack to themselves. Dipper, meanwhile, had to consciously remind himself not to shower in swim trunks, and so he was lagging behind when he first heard the voices.

Mabel let out a muted giggle, and ran back towards Dipper, quietly calling his name. His outreached hand caught hers, and she gave it a reassuring squeeze. She must have noticed her twin's slight hyperventilation, because she said with a reassuring voice "Relax, Dip. We're invisible, remember? I can't see a hair on your head, and neither can they."

Dip started to nod, before remembering his condition and sheepishly said "Yeah, I guess." Her hand tugged his, leading them past a group of hikers sitting by the trail, and onwards towards the town, specifically in the direction of the mall.

They cut through the parking lot, diving to one side roughly to avoid a car that, Dipper facepalmed in retrospect, _couldn't see us._ Mabel pulled him close, giving him a reassuring loving kiss on the cheek, before pulling him inside the doors after a gaggle of teenagers.

The warm, toasty feeling from his sister's body heat being pressed so close rapidly dwindled as a wave of fear hit him. There were hundreds, maybe thousands of people, and Dipper's mind reminded him that, yet again, he was stark naked. _Oh god, we're a banana peel and a bucket of water away from never being able to show our faces in public ever again ohgodohodohgod-_

Mabel's voice interjected again, as she said quietly but firmly _"Dipper!"_ He realized he'd been hyperventilating again, and ducking behind the mall map, took a few shuddering breaths. He could almost imagine her smile as he felt his sister's hand on his shoulder. "It's all right, brobro," she said quietly, "Nobody can see us, nobody's _gonna_ see us, so we might as well have some fun, yeah?"

Dipper sighed, and in a somewhat-sulky voice said "Fine, if you say-" He cut off as Mabel's unseen head said "Good!" before ducking down, and Dipper felt his knees go weak as she teased the head of his cock with her tongue. The healthy dousing of fear had shrunk Little Dipper to a pale shadow of his former self, but his twin's ministrations soon had him fully erect. He reached out a hand for stability, running it through her silky hair as her head bobbed on him for the second time that night.

He could feel his vision narrowing when suddenly the lovely, encompassing warmth vanished, leaving the cold breeze of the mall's AC to suddenly shock him into focus. He could feel his sister stand, and lean in to kiss him briefly on approximately his lips. It was technically the edge of his chin, but given that he'd almost poked his eye out earlier all by himself, Dipper wasn't going to fault his twin's aim.

Mabel had wrapped her arm around his waist, and was walking him forward hugged close alongside her as if they were out on the town. She was giggling, as quietly as she could manage, but even so Dipper could still see the occasional person turn, a slight look of confusion on their face at hearing a voice that didn't match a passerby.

"Dipper, my boy," she said, as if she was a statesman about to give a speech, "The rest of the evening is ours; We should _enjoy_ ourselves." The way she said _enjoy_ sent a little happy tingle back to Dipper's crotch, and he hugged her close to give her a quick kiss on her approximately-cheek. "So where-to first, then?" he said as he finally located her ear and gave her a quick hot nibble on her earlobe.

She muffled a squeal, and pulled his hand towards a lower area of the mall in lieu of pointing. Dipper remembered there had once been a Hoo-Ha's there, but repeated issues with apparently haunted animatronics stuffing security guards into the puppet suits had led to it's closure, demolition, and exorcism by an ordained priest. Since then a bookstore and a Yumberjacks had opened, but between them was a-

Dipper paused as Mabel tugged him towards the shop, before grinning like a madman and following her. She must have noticed, because she stopped short, cupping him briefly with a quick fondle, his blood surging southwards as she said breathily into his ear _"Last one there is a rotten egg!"_

Then she turned, giggling quietly as she ran in the direction of the shop, as Dipper chuckled and ran after his sister.


	3. Chapter 3

Dipper chuckled nervously and coughed as Mabel began rooting through a rack of clothing. They'd just ran into _Victoria's Common Knowledge_ , the lingerie store that Dipper was only familiar with thanks to a stack of dog-eared magazines and a single, very uncomfortable visit for a gift for Pacifica back when they'd still been dating.

He winced with the memory; The night he'd brought over the gift had been the night words had been exchanged, a glass of wine had been thrown, and Dipper went home single, unopened package under one arm. _However,_ he thought, looking in the direction of his sister as a poltergeist seemingly hummed quietly while flinging bras and teddies aside casually, _I think I came out ahead in the end. Hell, if I remember my sister right, she's about the same dimensions as Pacifica, if a bit more 'generous' in the right squeezable places; Maybe a regifting is in order?_

Striding over, he guessed where his twins nude, invisible ass was and grabbed. His effort was rewarded by a quickly-strangled squeak as he found his mark, Mabel gently slapping his hand away but reaching over to grab and stroke Little Dipper briefly before going back to the rack.

Dipper had become notably distracted by the attention, and let out a little impatient groan as she turned away. He took a step forward, pushing himself against her while letting out a little growl, but after a quick giggle she said sternly under her voice _"Dip!_ Just…just turn around or something; I want-I want to-Oh, just turn around already!"

He grinned, unseen, as he could tell she was getting flustered in that cute way he loved, but he obliged her; She couldn't see if he had, but he knew she trusted him to do so, and Dipper personally enjoyed sexy surprises, so he wasn't complaining either.

Plus, it gave him a chance to look around the rest of the store. There was a lone employee at the far counter near the entrance, but other than that the store was empty. The employee had his nose buried in a book, some kind of romance novel judging from the hairy shirtless scotsman on the cover, and as a result he must have assumed the store was empty. The mall muzak covered Mabel's rifling through the displays, but after another minute or so, he heard a low _"Dip? Dip, where are you?"_

He walked towards the sound of her voice, saying "Mabes, I'm over here," as he got close, and felt the breath go out of him as he bumped heavily into his twin. She fell, giggling as an armful of various lacy bits of clothes went everywhere, and Dipper snickered as well.

Apparently, they must have been louder than they thought, because a second later they heard a confused "Hello? Who is that?" from the counter. Dipper felt his eyes go wide, but Mabel's giggle went from laughing to mischievous, as she grabbed his arm and then his hand, pulling him after her as she ran with her armful of lingerie.

She had pulled them into one of the empty dressing rooms, pulling the door almost-closed so a passerby wouldn't see a floating bra and panty set, but not enough to make the employee think someone might still be in there.

Putting a hand on his shoulder, she firmly spun him around, saying "Ok, hold tight and the authentic Mabel-brand fashion show and striptease will begin in a minute."

Dipper felt his heart surge a bit at the second half, but after a minute of the sounds of struggling and clothing, he could hear a defeated "Uh, little help brobro?"

He turned, and took a second to comprehend what he was seeing. It looked like someone had made a loose ball of yarn out of silk ribbon, and tossed in some lacy black handkerchiefs for effect. After another second of comprehension, he realized what was going on and snickered, earning a disapproving _"Huff!"_ from Mabel. She had somehow managed to put on the strappy outfit upside down, and if Dipper's guess accurate, she had at least one arm through somewhere that was not an arm hole.

He took pity on his flailing sister, and helped her right the outfit. Before she got it finally situated, she spun him around again, saying "Thanks Dip, but there's a few finishing touches I can do by myself."

He turned, and could hear the rustling of paper, and several zipper noises as well as the sound of cloth sliding on skin, before a little timid voice said "Okay, you can look."

He turned, and felt his heart skip a little, before warming with loving sympathy for his sister. The outfit accentuated the edges of her curves wonderfully, as did the fishnet arm warmers, stockings and black kneehigh boots she'd conjured out of nowhere as well, but he felt barely a twinge of arousal; Evidently, seeing his sister's soft skin being hugged by the fabric was far more of a turn-on for Dip than seeing it suspended like it was held together for a display on fine strings and wires.

But the surge of sympathy was because he recognized the pose with the way her hips and shoulders must have been positioned; She was feeling self-conscious, unsure of what her brother was going to think or say, and his silence must have clued her in to his tepid response as she sadly let out a little shaky breath.

She started to turn, but Dipper reach out, caught her arm, and pulled her towards him. He had originally done so reassuringly, beginning to say "Aw Mabes, it's o-" before stopping.

While the invisibility had definitely crimped how much he was being aroused by what he could see, what he could _feel_ was another matter entirely. Feeling his sister's warm arm under the fishnet, where the curve of her ass met the base of the silk straps when he reached his arm around, _that_ was definitely making Little Dipper's day.

 _"_ _Oh,"_ he said, and he could feel her tense in confusion, but then shimmy up to him and press her body against his as she realized what was going on. "Well, Dipper," she said teasingly, "What do you think of this one?"

He growled appreciatively, trying to pull her towards him to sit on his lap, and by _sheer coincidence_ his erect cock as well, but she merely bent over slightly, rubbing herself teasingly along his shaft before standing up and beginning to pull off the outfit.

Again, she had Dipper turn and not peek; It was _far_ harder this time, but he managed, and after a minute more she tapped him on the shoulder again. He turned, letting out an appreciative low whistle; His sister had on a one-piece bathing suit, modest in coverage but risque in the thin, glittery choice of material, her nipples clearly erect underneath the reflective fabric.

He growled again, pulling her over towards him, but when she went to try and turn and squat again to teasingly rub herself along him, Dipper reached over, running a finger along where her crotch had been oh-so-nicely outlined by the swimsuit. She let out a gasp that shivered into a moan, and suddenly she stopped getting up, and sank back down to grind against him, hard, as she let out a growl of her own.

Their lips met, and Dipper enjoyed the sweet vanilla-bean taste as his other hand went to tease one of the excited nipples. She reached down, pulling the fabric to one side, and began rubbing her lips along and around the head of his cock, causing him to groan. He moved his hand to her thigh, and began insistently pulling her down towards him.

Mabel obliged, and he slid into her; Apparently she had been more turned on by this whole affair than Dipper had realized, because after just a minute of gentle rocking and him occasionally raising his hips slowly, he could feel her shudder and come, tensing around him.

Dipper was having a slightly harder time enjoying himself, as the material of the swimsuit, while beautiful and sexy as hell, had slid back to sandpaper itself against the side of Little Dipper. He reached up and pulled the straps off of his twin's shoulders, and she did the rest, shimmying it down to discard on the other side of the booth, as they sat back down on the cushioned bench.

"Guess we'll have to try some of these when Mabel is plain-old Visible Mabel," he said, looking at the stack of lingerie on the opposite bench a few feet away, and he could feel her nod, although she caught herself and responded with a little laugh and a quiet "Yeah, definitely."

He could feel her freeze along with him, though, when they heard the dressing room door next to them clatter open, along with the employee's voice shouting _"A-HA-uh? Dammit, I could have sworn I heard someone."_

Dip forced his breathing to a crawl as the footsteps approached and the door creaked open. The employee swore, itching his head as he looked around. Dipper could feel his breath catch in his throat in terror as he looked right at them, but then his gaze passed back to the discarded clothes, and he said "Man, I gotta clean up after some damn teens _again._ This blows."

Dipper started to let out a slow, whoofed breath, and Mabel did the same, even going so far as to shift and grind a little on him, restoring the erection inside of her that had been softened by fear.

Then the employee turned towards them, hand outstretched.

Dip choked down a whimper of terror, whipping his head around furiously before spotting the lone, guilty neon-teal thong he was unwittingly sitting on top of. There was no way his hand was going to avoid hitting _something,_ and Dipper tensed, as did Mabel, the employee bound to meet them at any second-

"BOO!" His sister leapt off of him, shoving the employee while screaming in his face. The employee responded with a high-pitched shriek, falling and scrabbling backwards in a cloud of lingerie and lace as Mabel laughed and ran past him. Dipper followed soon after, grinning at the shaky voice behind him echoing panickedly "What the _fuck?_ I thought they exorcised all the ghosts and haunted shit!"

The rest of the afternoon was spent mischievously. The twins spent a while in _Baskin-Floggins: 21 Flavors of Kink!,_ the little poorly-lit sex shop over on one end of the mall, and proceeded to lead a pair of employees in little circles around the shop by placing dildo breadcrumb trails around the sets of shelves. He could tell from how she paused that Mabel was eying several of the options hanging from the wall at one point, but he leaned in, running a hand along her suddenly-shivering back as he murmured "If you want the real McCoy, all you need to do is ask."

She gave a little appreciative murmur back, but then was distracted by an array of glitter pasties and edible body paint before going over to the condom rack to start arranging the packets to form little inspiring messages. Dipper let out a slight disappointed sigh at the lost opportunity, but then moved to help her with the condoms.

They then moved on to Yumberjacks, stealing a few incredibly-hot-and-fresh fries while Dipper quietly whimpered about the idea of boiling oil being _that_ close to his unguarded junk, a splash away from utter tragedy. Mabel was about to start rearranging letters on the menu sign when the PA crackled on across the mall. _"The time is now 8:45 PM. The Gravity Falls Mall will be closing in 15 minutes."_

His hand in hers, he could feel his sister shrug, and say in a slightly dejected voice "Well, ready to go home Dippingsauce?" He grinned, but tugged her slightly away from the exit doors they had started heading towards. "Not yet Mabes, not yet."

Dipper had to admit, he was putting on _far_ more bravado for his sister than he felt when he pulled her up alongside him, both of them standing on the raised, empty set of benches overlooking the escalators and the main mall entrance. People were streaming out by the dozens, and if Dipper had been visible, he was sure he would have caught fire and died of embarrassment.

 _But I'm not visible, now am I?_ He pulled Mabel close for a kiss, his hand running and teasing along her side as it descended, running along her hip, reaching behind to give her ass a quick playful squeeze, before drifting under to run a finger along her lips, almost tickling them.

His sister was tense at first, but after a minute the anxiety began to wear away, replaced by excitement and passion. She returned the kiss, one hand running along his sparse-but-still-there chest hair while the other reached down to give Little Dipper some additional stimulation that he did not need, but absolutely did not mind whatsoever.

She lifted a leg up and Dip responded by reaching underneath to hoist her up, where she slid onto him with only a little maneuvering around. He started to lose his balance, but they shared a little laugh as he caught himself, locking his mouth on hers for another kiss. Mabel had starfished around him with her limbs, and the weight was finally getting to Dipper, so he started to try and shift towards the bench.

The sensation wasn't doing his stamina any favors, however, but then his twin climbed off unexpectedly. Dip could still hear her panting as she moved slightly away from him, towards the bench. She murmured for him to come over, her voice desperate and flooded with needing lust. He followed the sound, and his outreached hands felt the hot, slightly-sweaty skin of her back, tracing down to the top of her butt, and maneuvering himself, he slowly, almost lazily pushed into his sister from behind.

Mabel let out a little _"Meep!"_ of pleasure, shivering and tightening on his cock, and as Dipper began to thrust into her harder and faster, he could feel her shudder and clench on him, arching her back upwards and grabbing his hands, directing one to grab and play with her breast as the other seemed to be trying to grab his hip to pull her deeper onto him.

She began pounding her hips downwards, the bench making a little quiet creaking noise that caused Dipper to be suddenly grateful that the mall was almost empty now, the muzak still blaring out over any errant sounds they made. His sister had released Dip's hands, as they roamed around her chest and reached down to toy her clit and run wet fingers up around her nipples and to her suckling mouth.

Then, with another building groan of pleasure, Mabel grabbed his hips with her hands like before, and her pace slowed as he felt her clench in another orgasm, a shuddering happy _"Aw-oh-ohgodDip-oh"_ moaned out. She slowed to rest on him, a final happy aftershock sending her brother over the edge, and he came as well with an audible, drawn-out groan. Mabel let out an appreciative hum of pleasure at the sensation, drawing back to give him a kiss, before finally breaking apart as they saw the last handful of visitors approaching the door below them.

"As much fun as I'm having, I'd rather not be stuck here overnight." Dipper nodded at her words, and they broke apart, giggling quietly as they ran down the frozen escalators and shadowed the people leaving out the automatic doors.

Dipper found Mabel's hand, and they began walking towards the end of the lake they'd stashed their clothes in, when there was a rumble of thunder, and a few fat raindrops began to fall.

 **FIN.**


	4. Chapter 3 Alternative Ending

Mabel had pulled them into one of the empty dressing rooms, pulling the door almost-closed so a passerby wouldn't see a floating bra and panty set, but not enough to make the employee think someone might still be in there. Dipper had wrinkled his nose in annoyance as he heard a pair of girl's voices from a few dressing rooms down. They were quiet, full of giggles, and the pungent skunk-stink he recognized made him roll his eyes and try not to cough.

She murmured "All righty, Dip, turn around for this one." He did as he was bidden, looking towards the wall, and could hear the rustling of paper, and several zipper noises as well as the sound of cloth sliding on skin, before a little timid voice said "Okay, you can look."

He turned, and felt his heart skip a little, before warming with loving sympathy for his sister. The outfit accentuated the edges of her curves wonderfully, as did the fishnet arm warmers, stockings and black kneehigh boots she'd conjured out of nowhere as well. However, while it was hard to appreciate the lingerie on an invisible form, Dipper could hear her gasp and turn. He stepped forward to peek over the top of the dressing room door, and groaned.

The two girls had disappeared, leaving their still-lit blunt behind in a trash bin. Some old discarded receipts had taken objection to the ember present in their midst and burst into small, merry flames. The trash can was metal, and Dipper was fairly certain it couldn't spread, but the fire was creating a visible grey ribbon of smoke upwards.

"Oh _shit,"_ the twins said in unison before the sprinkler systems snapped on, alarms blaring as they could hear the employee at the counter for the lingerie shop begin swearing and running for the exit.

Mabel's head was already becoming visible as the magical waterfall's effect washed away, and Dipper held up a splotchily-visible arm with a groan as well. After a couple tense seconds, his sister hissed _"Go! Go!"_ at him, waving her hand as he sprinted into the shop, feeling a full-body blush fighting against blood-drained embarrassment as he realized most or all of his nude form was probably visible flopping about.

He screeched to a stop behind an abandoned kiosk as the sprinklers continued their merry rain. Mabel was right behind him, her black sheer teddy slicked to her form, although the water now just accentuated the black rubbery material of the boots. Dipper could feel himself starting to feel a warmth down below in spite of himself, but snapped his eyes back to the entrance. There was a huge crowd just outside the door, the refugee customers and employees of the mall shops.

He groaned in desperate hopelessness. "Man, Mabes, we'll _never_ make it out without getting spotted."

His mind began to whir with possibilities for breaking a window and rappelling to safety with some makeshift rope, when Mabel grabbed his shoulder, spinning him as she pointed.

"Look, where the Hoo-Ha's was; An emergency exit!" Sure enough a looming grey double-door was visible, tucked behind a trio of other little temporary kiosks, and the small shrine erected to keep the former area of the damned animatronic amusements sanctified and uncursed.

They ran for it, with Mabel shooting Dipper a glance. Even mostly-flaccid, she was very much enjoying seeing his "Little Dipper" again, and from how it twinged when he shot her looks, he was appreciating it too.

They came up short on the door, screeching to a stop in the puddles. Dipper just strode towards the door, but Mabel still stood, and in a little playful voice said "Where're ya going, Dip Dot?"

Dipper turned, confused, and then swallowed as he saw she had straddled a massage chair, her boots on either side matching the fake leather of the seat. As he watched, she winked saucily at him, leaning back to stretch and lift an arm lazily over her head.

 _You know what? Fire Department responses for stuff like this usually take a while._

He grinned, striding over to his sister to grab her hand, lifting her up and hugging her tight as his lips engulfed hers. She sighed, giggling, and he could still taste a trace of the vanilla-bean lipgloss as he sucked on and released her lip. Mabel pressed her damp form against his, a hand running over the top of his thigh before wrapping around his cock and giving it an appreciative pump.

Then she broke the kiss, cackling as she pushed him gently to sit and lean back in the massage chair. Dipper did, smiling and sighing as she straddled him, rubbing her warm, wet lips along his shaft while she trailed kisses up his chest. Dipper groaned, pleading without words in the sound, and his hips rose to meet hers.

Mabel kissed him on the nape of his neck, the heat causing him to shudder as she drew back and the cold sprinkler water froze the spot. She then lifted herself up, reaching a hand down to guide her before sitting to settle on his shaft, her hips bumping against his as he filled her.

She moaned, leaning backwards to pull his hands up to her breasts. Dipper nudged the cups of the teddy aside, thumbs playing with the nipples as she scratched her fingers down his chest. Dipper's hips rose to meet hers, and Mabel groaned, slowing and grinding against him between sets of lifting up to settle on and around his member. When he rose forward and gently nibbled one freed nipple, she had gasped, tightening around him, and letting out a happy growl.

A few more minutes, and she had come again, harder this time as Dipper's breath became ragged. Mabel dismounted, quickly ducking down to suck on his cock, the sudden shift from heat of her body to cold water to the heat of her mouth causing Dipper to cry out, clenching the handrests of the massage chair in a deathgrip as he came into his twin's mouth.

Mabel just made a happy hum around him, causing him to twitch again inside her mouth, and with a final suck and stroke of her tongue, she swallowed, and gave the head of his Little Dipper a kiss, before kissing him on his birthmark.

There was a distant sound of voices, and in a flash Dipper and Mabel were standing, the afterglow fading. Dipper looked towards the grey door, abruptly aware that there might be someone, anyone, a whole _crowd_ of anyones outside.

His sister noted his worried expression, and her eyes came to rest on one of the racks at a kiosk with an evil grin. Dipper sighed, letting out a defeated groan, and then grabbed a costume

They burst through the door into the bright sunlight of the dilapidated area behind the mall. They were alone, and Dipper sighed in relief. He pulled awkwardly at the wet, far-too-small blue sailor scout tunic and skirt; They didn't have a lot of options, and he had thoroughly shot down Mabel's joking proposal when she held out the Slave Leia costume from the Summerween costume rack.

 _Although, that gives me all sorts of ideas for fun later this summer._

He followed his sister as she ran for the treeline, Dipper appreciating the occasional flashes he got up her identical undersized pink skirt as they ran towards their clothes and the location of the enchanted invisible waterfall.


End file.
